The present embodiments relate to a system for electromagnetic excitation of an object under examination during magnetic resonance tomography with an RF device.
A conventional magnetic resonance tomography apparatus may have a magnet for generating a static magnetic field and also gradient field coils for generating variable magnetic gradient fields in all three spatial axes, which may be superimposed on the static magnetic field. The hydrogen nuclei aligned in the magnetic fields are excited by an RF device for generating RF excitation signals in the form of pulses and antennas for emitting these pulses into a volume in the magnetic field in which the sample is located. The density and the ambient conditions of the hydrogen nuclei in the sample are determined via an RF response signal that the hydrogen nuclei emit because of precession in the magnetic field as a response to the excitation pulses. The RF response signal is captured by antennas and processed in the RF device. Body coils that surround the volume with the sample may be used both as transmit and receive antennas.
In order to increase the receive sensitivity for small objects under examination (e.g., during examination of limbs or of the head), which only partly fill out the sample volume, it is known, for example, from U.S. Pat. No. 4,825,162 to dispose a plurality of receive coils directly on the object under examination. The receive coils are disposed overlapping so that the signal of a neighboring coil is just canceled out in a selected coil. The signals are “orthogonal” to one another and may be processed independently of one another for a volume of the object under examination lying therebelow in each case. To decouple next-but-one neighbors, preamplifiers with low-impedance inputs are provided.
From publications WO 2008/078239 A1 and WO 2011/054923 A1 it is additionally known (e.g., through geometrical arrangement and extensions on the coils that project sideways from the coils and in each case overlap with an extension of the coil-after-next) to also achieve a suppression of the interaction with the next-but-one neighbor in each case. This suppression is also effective for the use of the coils as transmit antennas for the excitation signal.
The respective arrangements suppress the neighbor-neighbor interactions in each case for a specific geometry (e.g., for a flat arrangement). In some cases, however, it is also necessary to arrange the coils spatially around an object under examination (e.g., a knee). In addition, the object under examination, through dielectric and magnetic properties, influences the electrical and magnetic field distributions, so that the signals from the neighboring coils are not completely suppressed.